Compression artifact
Compression artifacts are recognizable disturbances in digital images, videos or sound recordings. They arise from the lossy compression of the output data.
In principle, errors are always present in data that is compressed in a lossy manner. However, if these errors reach such an extent that they are noticeable to a user, one speaks of compression artifacts.
Lossy compression removes some of the information from the data so that it can be reduced so that it occupies the desired storage space or can be transmitted at a certain data rate. If the specified storage space or the data rate are selected too low, it is no longer possible to reproduce the data true to the original, and the quality deteriorates ( rate distortion theory ). The more data is compressed, the more information is lost and the more clearly compression artifacts occur.
Causes of Artifacts
- high compression rates
- Implementation error
- Errors in the psychoacoustic or psychovisual model
- when editing: overdrive digital filters / wrong parameter selection
- when editing: multiple compression / decompression
Pictures and videos
Images that are lossy compressed (e.g. as JPEG ) can have the following artifacts:
- Block artifact , clearly visible block formation,
- Overshoot (also ringing ), in areas with high contrast "waves" appear in the image,
- Posterization (also banding ), erratic course of brightness,
- Alias effect or moiré effect , if changes in color or brightness occur too closely to one another,
- Blurring , by removing high frequency components,
- Gibbs' phenomenon , a small area around an object with high contrast that clearly stands out from its surroundings
Sound recordings
With lossy compressed audio data (for example with MP3 ), compression artifacts become noticeable as (undesired) noises. This includes
- Telephone- like sound by removing high frequency components ( low-pass filtering ),
- Pre-echo: clinking metallic artifacts are audible in front of a sudden loud noise (e.g. drums)
- Post-echo: clear artifacts can be heard after a sudden noise
- alternately washed out sound, lack of brilliance , especially in highs and lows, as well as with certain instruments (cymbals, alternately washed out depending on other frequency components, hi-hat)
- Rattling (typical for MP3 when the compression is too high)
- inappropriate volume changes
- Change of stereophony , reduction of the spatial impression